The present invention relates to the preparation of maleic anhydride by catalytic oxidation of butane. In particular, the present invention is directed to improving the lifetime of the catalyst used during the production of maleic anhydride by catalytic oxidation of butane. In addition, the present invention is directed to a novel catalyst system comprising an intimate mixture of component containing a catalytic material and a component containing noncatalytic material capable of extending the lifetime of the catalyst.
Maleic anhydride is a commercially available chemical having significant utility throughout the world. It can be used alone or with other acids in the manufacture of alkyl and polyester resins. It is a versatile chemical intermediate also useful as a monomer to produce various copolymers such as the copolymers of maleic anhydride and vinyl acetate. Significant quantities of maleic anhydride are produced each year to satisfy these needs.
Typically, maleic anhydride can be produced by oxidizing hydrocarbons such as butane, butene, 1,3-butadiene and benzene. The oxidation is carried out in the presence of an oxidation catalyst. Vanadium-phosphate catalysts having the general formula V.sub.a P.sub.b O.sub.x are especially active to catalyze the vapor phase oxidation of hydrocarbons to maleic anhydride. For example, the high surface area catalyst described in U.S. Pat. No. 4,123,442 have been utilized in the oxidation of butane to maleic anhydride. In addition, promoted vanadium phosphate catalysts such as those described in U.S. Pat. No. 4,371,702 have also been utilized in the production of maleic anhydride from butane. In general, these catalyst comprised of vanadium, phosphorus and oxygen, have an intrinsic surface area in the range of about 7-60 square meters per gram, a phosphorus to vanadium atomic ratio in the range of 0.9-1.8:1 and an average vanadium valence in the range of 3.9 to 4.6.
The oxidation process is usually carried out in a fixed bed and the catalyst is contacted with a hydrocarbon under reducing conditions. Typically, the oxidation is carried out at about 300.degree.-500.degree. C. In practice, the oxidation feed is fed to a fixed bed tube reactor containing the catalyst material. Shortly after the hydrocarbon is introduced into the reactor, it begins to oxidize rapidly and creates a hot spot in the catalyst bed. As the time of reaction on the catalyst increases, the activity and selectivity of the catalyst decreases. This is generally termed catalyst aging. As the catalyst ages, the selectivity of the catalyst to maleic anhydride drops precipitously while total conversion of the butane remains constant or increases. When the selectivity to maleic anhydride drops below the acceptable level, the catalyst must either be replaced or regenerated. For examples of typical catalyst regeneration, see U.S. Pat. Nos. 4,020,174; 4,123,442; 4,081,460; 4,006,168 and reissue Patent No. 0030430.
While regeneration of the catalyst is a viable alternative to replacement, it should be highly desirable to increase the lifetime of the catalyst so that regeneration can be delayed as long as possible. Applicants have found a relatively simple and economical procedure for increasing the lifetime of such catalyst without affecting the selectivity of the catalyst to the production of maleic anhydride.